Kind of hazy out there? Blame SAL

Beachgoers get a look at the setting sun on Fort Myers Beach on Monday 8/18/2014. A Saharan Air Layer has drifted over the area causing the sun to look more muted. There are small sand particles from the Sahara Desert in atmosphere. (Photo: Andrew West/The News-Press Andrew West/ The News-Press)

Beachgoers get a look at the setting sun on Fort Myers Beach on Monday 8/18/2014. A Saharan Air Layer has drifted over the area causing the sun to look more muted. There are small sand particles from the Sahara Desert in atmosphere. (Photo: Andrew West/The News-Press Andrew West/ The News-Press)

Beachgoers get a look at the setting sun on Fort Myers Beach on Monday 8/18/2014. A Saharan Air Layer has drifted over the area causing the sun to look more muted. There are small sand particles from the Sahara Desert in atmosphere. (Photo: Andrew West/The News-Press Andrew West/ The News-Press)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

A Saharan Air Layer has made it's way over the Atlantic Ocean to South Florida. Small dust particles from the Sahara desert in Africa are in the atmosphere creating a haze over the area. (Photo: Andrew West/news-press.com)

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The sun rises behind a hazy sky at Matlacha Pass on Monday. Hazy conditions since Saturday in Southwest Florida have been caused by a Saharan Air Layer, which is extremely dry, dust-filled air that drifted across the Atlantic Ocean from Africa.(Photo: Kevin Lollar/The News-Press)

Fifty years ago, Nat King Cole was singing about "those lazy, hazy, crazy days of summer," and, while Southwest Florida might not be lazy or crazy this week, it's definitely hazy.

Blame the haze on SAL, a Saharan Air Layer, which forms when dry air and dust rise from Africa and roll with the trade winds across the Atlantic.

A large SAL reached Florida on Friday, said meteorologist Jeral Estupinan of the National Weather Service in Miami.

"At that time, it was only noticeable by instruments and maybe with the naked eye if you were familiar with it," Estupinan said. "Saturday and Sunday, slowly and surely, it started becoming thicker. The dust is like a white haze, not like the typical haze we see in the Southeast in places like the Smokey Mountains, which is bluish."

Another interesting thing about Saharan dust is how it can affect sunrises and sunsets, said meteorologist and Jason Dunion of the National Oceanic and Atmospheric Administration's Hurricane Research Division.

Sunlight is made up the colors of the rainbow, red, orange, green, blue, indigo and violet, with the red end having longer wavelengths than those at the blue end.

Atmospheric debris (dust and ash) blocks light as it passes through the atmosphere; colors with short wavelengths are the first to disappear.

When the sun is high in the sky, sunlight doesn't pass through much atmosphere, so the debris doesn't block much light.

But when the sun is close to the horizon, light passes through more atmosphere, and the debris blocks the short-wavelength blue end of the spectrum, resulting in orange and red sunrises and sunsets.

"Saharan dust particles are a little bigger and tend to scatter long wavelengths, too," Dunion said. "So at sunrise and sunset, you see more yellows and get an eerie glow, not brilliant red. If you have clouds in the sky, you could get some neat sunsets."

SAL can also affect the weather.

"It's a thunderstorm suppressor," Dunion said. "Right now, it's in a layer one to three miles above Fort Myers, and the air is super-dry, so any thunderstorms that run into that dry air will collapse.

"But the weather is still sultry, hot and sticky, still summery, so without storms to cool things down, we can get some of the hottest days in South Florida."

The current SAL left the west coast of Africa 10 to 12 days ago and now stretches from Africa to southern Texas, Estupinan said.

"It's extensive, but it's not evenly formed," he said. "There are areas with more dust than others. We're in a patch with a lot of dust that extends from a little west of Florida to thousands of miles into the Atlantic."

Tropical waves, the seeds of tropical storms, form off the west coast of Africa, but dry air from a Saharan Air Layer can prevent a wave from becoming a storm.

As strange as it might sound, SALs are the result of thunderstorms over the Sahara.

During Africa's summer monsoon season, thunderstorm winds blast tons of dust into the air, and much of that dust rises into the atmosphere.

"With thunderstorms day after day, you get dust, dust, dust," Estupinan said. "With the easterly winds, there's no place to go but westward across the Atlantic."

This week's SAL should be gone by the weekend.

"We've had a bunch of SAL outbreaks this year," Dunion said. "It seems like they're bigger, and they're reaching farther west than usual. We don't know why. That's what makes it interesting.

"Looks like late next week, we'll get another round. And so it continues."

— Kevin Lollar

Hurricane forecast

Meteorologists are watching a tropical disturbance in the eastern Atlantic Ocean that is expected to strengthen very little this week.

Forecasters at the National Hurricane Center give the disturbance a 10 percent chance of forming into a system within the next 48 hours and a 30 percent chance of developing into a system by the weekend. The disturbance is moving slowly westward and is about midway between the Cape Verde Islands and the Lesser Antilles.

The hurricane season has produced two named storms — Hurricane Arthur, which struck North Carolina in July, and Hurricane Bertha, which did not impact the U.S. mainland. Experts with the National Oceanic and Atmospheric Administration and Colorado State University lowered their forecasts for the remainder of the season, noting that conditions in the Atlantic Ocean and Gulf of Mexico, overall, are not conducive to hurricane formation.

A typical year, based on weather records dating to 1950, has 12 tropical storms, of which seven become hurricanes. A tropical storm has sustained winds of 39 mph; it becomes a hurricane when its winds reach 74 mph.